1. Field of the Invention
The present invention relates generally to processes for making fluorinated amorphous carbon films and more specifically to processes for making fluorinated amorphous carbon films for use as interlevel dielectric layers.
2. Description of Related Art
Advanced integrated circuits for semiconductor devices having higher performance and greater functionality are often characterized by decreasing device feature geometries. As device geometries become smaller, the dielectric constant of an insulating material used between conducting paths becomes an increasingly important factor in device performance. Reducing this value advantageously lowers power consumption, reduces crosstalk, and shortens signal delay for closely spaced conductors.
Silicon oxide (SiO.sub.2) has long been used in integrated circuits as the primary insulating material. With a dielectric constant of approximately 4, SiO.sub.2 has the lowest dielectric constant of all inorganic materials. Adding small amounts of fluorine into the SiO.sub.2 film can lower this value to 3-3.5. Further reduction, though, requires use of organic materials. Fluorocarbon-based polymers have been recognized as potentially attractive low dielectric constant materials. For example, polytetrafluoroethylene (PTFE) has a bulk dielectric constant of around 2, essentially the lowest of any non-porous, solid material. However, such fluorocarbon polymers generally have limited thermal stability, with decomposition occurring around 250-300.degree. C., making them incompatible with many semiconductor fabrication processes. They generally have limited mechanical stability, as well.
It is known that the dielectric constant of a fluorocarbon film decreases with increasing fluorine concentration. (See, for example, S. Takeishi, et al., J. Electrochem. Soc. Vol. 144, p.1797 (1997).) It is also known that fluorine concentration is one of the factors affecting the thermal stability of such films, the stability decreasing with increasing fluorine concentration. The challenge, then, is to provide a fluorocarbon film as an interlevel dielectric layer that simultaneously meets the criteria of low dielectric constant, thermal stability, and physical durability.
One attempt at a polymeric fluorocarbon layer is described in U.S. Pat. No. 5,244,730, "PLASMA DEPOSITION OF FLUOROCARBON." The fluorocarbon film, prepared by plasma enhanced chemical vapor deposition, is reported to have a maximum dielectric constant of about 2.5, a F:C ratio of about 1:1 to 3:1, and is reported to be thermally stable to about 350.degree. C. However, the fluorocarbon layer does not adhere sufficiently to typical semiconductor substrates. To improve adhesion, a thin layer of silicon or a metal silicide is introduced between the substrate and the polymeric film, such that a region containing a high density of Si--C bonds is formed. (See U.S. Pat. No. 5,549,935, "ADHESION PROMOTION OF FLUOROCARBON FILMS".) This adhesion technique, however, does not address the adhesion of the polymeric film to subsequently deposited layers above the film.
Another attempt at providing a fluorocarbon dielectric layer is reported in U.S. Pat. No. 5,698,901, "SEMICONDUCTOR DEVICE WITH AMORPHOUS CARBON LAYER FOR REDUCING WIRING DELAY" by Endo. Fluorinated amorphous carbon films, produced by plasma-enhanced chemical vapor deposition using CF.sub.4, C.sub.2 F.sub.6, C.sub.3 F.sub.6, C.sub.4 F.sub.8, or CHF.sub.3 as a fluorine precursor, have dielectric constants between 2.3 and 2.6, fluorine content between 48% and 56%, and are heated to temperatures between 300 and 470.degree. C. before beginning to decompose. Problems of adhesion and delamination need to be addressed, however, to be able to integrate the fluorinated amorphous carbon films of Endo into semiconductor devices. The approach of Endo to these problems involves multiple layers. An adhesion layer is provided by varying the fluorine content in the film such that the lower edge of the film near the substrate is pure amorphous carbon. Further, a buffer layer is introduced between the film and the elements of the semiconductor device to suppress gas discharge out of the film on heating. Finally, a transition layer between the buffer layer and the film, with composition varying gradually between that of the buffer layer and that of the film, is used.
Thus there is a need for a simpler approach to promoting adhesion of a fluorocarbon film to materials commonly used in semiconductor devices. It would be desirable if an adhesion layer also serves as a capping layer to suppress gas discharge out of the film on heating. It would be desirable for the fluorocarbon/adhesion layer stack to withstand rigorous adhesion and thermal testing. In addition, it would be desirable to provide a single continuous process for producing the fluorocarbon interlevel layer in a semiconductor device. It would further be desirable to produce films with particular F:C ratios by use of advantageous fluorocarbon precursors.